The 33-year-old heir to the throne attended a gala dinner for Child Bereavement UK, marking 21 years since the charity opened.
During an emotional speech, the dad-of-two spoke about what he had learned about grief since losing his mother in 1997, and since becoming a parent himself. He said:
'Grief is the most painful experience that any child or parent can endure.
'What I understand now is that grief is the most painful experience that any child or parent can endure,' William said in an emotional speech.
'Twenty one years ago last month, my mother attended the launch of the Child Bereavement Charity.
'Fifteen years later, I was honoured to be invited to become patron of Child Bereavement UK to continue my mother's commitment to a charity which is very dear to me.
'What my mother recognised back then - and what I understand now - is that grief is the most painful experience that any child or parent can endure.
His Royal Highness also met with Take That singer Gary Barlow. The charity is close to the singer's heart after he and his wife had a stillborn daughter, Poppy, in 2012.
'But my mother was determined to help those in need and she would have been immensely proud - as I am - of all that Child Bereavement UK has achieved these last 21 years.
The duke added, 'When many people slink away at the sight of a friend's bereavement, Child Bereavement UK staff embrace strangers at the darkest moment in their life.'
Princess Diana helped her best friend, Julia Samuel, launch Child Bereavement UK back in 1994, and the charity founder is now a godmother to Prince George.
Prince William described Princess Charlotte as 'very ladylike', but big brother George is more 'lively'
The Duke of Cambridge went on to say that fatherhood had helped him appreciate the charity's work all the more. He praised his young family, describing Princess Charlotte, who was born in May 2015, as 'very ladylike' in contrast to his eldest, Prince George, as 'lively' (a phrase all new parents will be familiar with) - and if that wasn't enough, the family's pet dog Lupo is apparently an expert at destroying the lawn!
Earlier in the day William also had a chance to re-visit his old college, St John's in Cambridge, where college master Christopher Dobson gifted him a children's book, which the former-RAF pilot promised to read to his little ones.
Prince William and his young family recently moved from their home in Anglesey, Wales, to Anmer Hall in Norfolk, and the Duke now works for the East Anglian Air Ambulance in Cambridgeshire.